Tag Archives: solutions

1.) New images taken by the Gaia SM (Sky Mapper) CCDs were just published by ESA. There are images of the cluster NGC 2516, the spiral galaxy Messier 94 and Cat’s Eye Nebula available. Enjoy the new images here:

The spiral galaxy M94 as seen by two SM CCDs. You can clearly recognize the gap between the two CCD chips. Most likely this is the last time that images of these kind of extended objects are downloaded, as Gaia will observe only point sources during nominal observations.

2.) There are some more news about the status of Gaia available now in an article by my colleague Stefan Jordan in the German journal ‘Sterne und Weltraum’. The commissioning is ongoing quite well and will continue until May at least.

Two issues are mentioned for the first time (I think) in this article:

There was a problem with one thruster of the MPS, the Micro Propulsion System. It was delivering much more thrust than desired. But the issue was solved very quickly by a recalibration of the nitrogen supply mechanism.

A contamination issue was detected for one of the mirrors reducing the throughput of the affected telescope by more than the half. The contaminant has still to be determined, but the contamination was already successfully cleaned by heating this particular mirror. Investigations are ongoing to identify the contaminant to make sure that a similar scenario cannot happen again. It is not ruled out that there is a correlation to the stray light issue (more information about this here https://hvossgaia.wordpress.com/2014/02/12/gaia-status-update-after-one-month-of-commissioning/). Parts of the thermal tent are most likely heated by the stray light and an increased outgassing of the paint used for the thermal tent could be an explanation for the contamination seen. But other scenarios are still under investigation. Most likely we will learn more about the issue after next Monday, when the sun aspect angle of Gaia is changed by three degrees to reduce the stray light effect. Let us keep the fingers crossed that this manoeuvre will yield the desired results. 😉

All these are small issues normal for the initial phase of a space mission. In Germany we call it ‘Kinderkrankheiten’ – these kind of not so serious illnesses that small children are suffering from. Solutions were found and in most cases already applied. Overall, the mission progress is quite impressive.

Several important systems and subsystems of the satellite were already tested – among them the Micro Propulsion System (MPS), the Phased Array Antenna (PAA) for the downlink of the data and all of the 106 CCDs in the focal plane. All these systems are working now as expected.

One unexpected feature seen in the test observations are temporary high background signals due to stray light. This stray light is entering the telescope openings in the thermal tent around the payload and reaches unfortunately the focal plane with the CCDs. This is obviously an unwanted issue as it influences the error budget of some measurements.

Gaia model at ESOC. Currently it is assumed that light from the Sun (which is below the sunshield in 45 deg inclination from the spin axis of the satellite) is diffracted on the edge of the sunshield – sometimes towards the two telescope openings on the top of the satellite.

How is it possible that stray light can enter the telescope openings? Several theories were discussed, the leading theory is that light from the Sun is the origin of the stray light as this light shows a periodic behaviour in agreement with the 6 hour spin period of the satellite. Gaia has a sunshield supposed to protect the payload from the Sun light all the time. Is something wrong with the sunshield? The favoured explanation of the experts is that light from the Sun is diffracted at the edges of the sunshield – sometimes unfortunately towards the telescope openings. There it enters the payload module, is reflected on the wall of the thermal tent and reaches somehow the focal plane. 10 mirrors, some more optical elements and other reflective components are installed inside the thermal tent. The very complex analysis of the situation is still ongoing.

Gaia in launch configuration. The sunshield is folded and is forming the outer limitation of Gaia towards the inner payload fairing.

Nevertheless, there is already a solution for this issue under preparation. The orientation of Gaia towards the Sun will be changed a little bit. The inclination of the spin axis in relation to the Sun will be moved from 45 deg to 42 deg to reduce the chances of diffraction of Sun light on the edges of the sunshield. This manoeuvre is planned for early next week after some intensive planning. The experts of ESA, Astrium and our DPAC payload experts group are waiting eagerly to analyse the data collected after this manoeuvre is executed – hopefully with the expected positive result of lower background levels in the images.

The commissioning (testing) of Gaia will be ongoing for a few more months. Many things still have to be optimized and calibrated to allow the high-precision measurements of the nominal observations during the next 5 years or more.

Note: If you have comments or questions please let me know. I may be able to give some replies in the comment section! 😉